Transparency of aid activity is being recognized to be one of the key areas whereby aid effectiveness can be improved. In this paper, we propose an index to measure and rank donors on the transparency of their aid activities. The Transparency Index rates 31 bilateral and multilateral donor agencies on six measures of transparency. We find that being a member of the IATI is a powerful signal of a donor being more transparent across most other dimensions as well. We find no relationship between transparency and donor aid volumes. Overall IDA and Australia are identified as the most transparent donors, while Korea and IDB Special Fund are the least transparent.